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The only examples I found of nonprincipal circle bundle are nonorientable, like the Klein bottle that is an S^1 bundle over S^1 which is not principal and nontrivial. That makes me ask the question.

Is it true that every orientable circle bundle is principal?

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    $\begingroup$ This is essentially true because $Diff^+(S^1)\simeq U(1)$, so $BDiff^+(S^1)\simeq BU(1)$ from the fibration $Diff^+(S^1)/U(1)\to EDiff^+(S^1)/U(1) \to EDiff^+(S^1)/Diff^+(S^1)$ with contractible fibers. $\endgroup$
    – Ian Agol
    Commented Oct 6, 2013 at 17:03

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Yes,it is true.You can find this result from "Geometry of differential forms" by Morita (Page 241)

PROPOSITION 6.15. Every oriented $S^1$ bundle admits the structure of principal $S^1$ bundle.

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